A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications. In a proton exchange membrane (PEM) fuel cell, the electrolyte is a solid polymeric membrane which is electronically insulating but ionically-conducting.
The principle component of a polymer electrolyte fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, typically comprising a platinum-based electrocatalyst. An electrocatalyst is a catalyst that promotes the rate of an electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.
The MEAs can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane. Subsequently, gas diffusion substrates are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion substrate adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
Typical gas diffusion substrates include substrates based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan), woven carbon cloths (e.g. Zoltek® PWB-3 available from Zoltek Corporation, USA) or non-woven carbon fibre webs (e.g. Optimat 203 available from Technical Fibre Products, UK). The carbon substrate is typically modified with a particulate material either embedded within the substrate or coated onto the planar faces, or a combination of both. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE).
U.S. Pat. No. 6,511,768 discloses a gas diffusion substrate comprising a graphitised fibre web structure. The web is manufactured by taking a web structure made from polyacrylonitrile (PAN) fibres, and oxidising and graphitising the web. The graphitisation step is carried out at temperatures from 1500 to 2500° C. A high temperature treatment of this kind requires significant energy input, which adds to the cost of the manufacturing process.
EP 791 974 discloses a continuous manufacturing method for preparing gas diffusion substrates that does not use a high temperature graphitisation step. Carbon black is mixed with PTFE, and carbon fibres are coated with PTFE. The carbon black/PTFE mixture and the coated carbon fibres are mixed to form a slurry which is deposited onto a moving mesh bed. The deposited layer is dried, forming a gas diffusion substrate.